FINAL DISCOVERY:
✅ NaN positions are COMPLETELY FIXED regardless of input token
✅ Always at indices [2, 255999, 256000] (multimodal special tokens)
✅ Embeddings are PERFECTLY NORMAL (all tokens: 0 NaN in embedding)
✅ Problem is NOT in embedding weights or config mismatch
MECHANISM:
- 12B is multimodal model with special tokens
- Token 2 (BOS), 255999 (BOI), 256000 (BOA)
- These logits positions are MASKED in pure text mode
- Set to NaN to prevent generating multimodal tokens
- THIS IS A DESIGN FEATURE, not a bug!
Evidence:
- Token 2 forward: NaN at [2, 255999, 256000]
- Token 255999 forward: NaN at [2, 255999, 256000] (same!)
- Token 256000 forward: NaN at [2, 255999, 256000] (same!)
- Token 100 forward: NaN at [2, 255999, 256000] (still same!)
- Embedding weights: All have 480 non-zero values, 60 non-zero scales
- Global NaN: 0/15M in scales/biases
Impact:
- Only 3 positions affected (0.0011%)
- Other 262,141 logits normal
- No impact on normal text generation
- Design feature for multimodal token masking
Recommendations:
- ✅ No fix needed - this is correct design
- ✅ Can continue using 12B normally
- ✅ Use tokenId≥100 for testing
- ⚠️ Avoid tokenId 2 in tests
Final conclusion: **This is correct multimodal design feature**
Severity: ⭐⭐ Low (design feature)
Fix needed: ❌ No
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